A computer gadget the size of a flashlight can make life easier for anyone involved in house hunting, building or remodeling.

The remodeling aid is Seiko's Home Contractor Conversion Computer, and Mark McIntire, vice president of the New York Design Center, wrote in the current issue of House Beautiful that it is a combination measuring device and building materials estimator.McIntire, a frequent lecturer at the Parsons School of Design and the Fashion Institute of Technology, wrote that it will read out square footage and cubic volume to estimate required amounts of paint, wallpaper, carpeting and tiles, plus BTU-HR needs for heating and cooling.

To use the device, which lists for $89.99, place its base against one wall, aim it across the room toward the other wall and give it a little squeeze. Instantly the easy-to-read display flashes a measurement and, when you touch the right button, records that number as length or width.

Repeat this process on the other walls, then take a reading from floor to ceiling. Push two more buttons and you get area and volume in metric or U.S. measurements. Ten seconds later, when the self-timer clicks off, these numbers are stored for future recall.

That is how you use the simple measurement computer. Switching to the Conversation Computer requires more thought.

For example, after punching in your bedroom dimensions, you might want to compute the number of floor tiles it would need. The computer is preprogrammed to estimate based on 10-by-10 inch tiles. Reading of the manual reveals a series of multipliers for tiles of various sizes and for 15-by-15 inch carpet tiles.

Here are some helpful tips:

- Having basic estimates before making a store visit is advantageous. Anyone contemplating renovation, new construction or home buying will find the pros of this relatively inexpensive unit far outweigh the conversion-process cons.

- If you want carpeting to flow down halls, up stairs and through various rooms, you can spare yourself the tedious tape measuring-calculating process. Once your computer has a total yardage estimate, you divide that number into your budget to find out how much per yard you can spend on carpeting.